The present invention relates to a semiconductor laser having a multilayer structure and more particularly a semiconductor laser having an interface between two layers which is configured as a diffraction grating.
Such semiconductor lasers are known as DFB (distributed feedback) or DBR (distributed Bragg reflector) lasers and are disclosed in European Pat. 0,173,269. Such lasers have a wavy interface between two layers so as to form a grating at which light emitted upon stimulation is diffracted in such a way that first order observation causes light to propagate at an emission wavelength .lambda.0 which corresponds to twice the grating period.
The difference between known DFB lasers and a DBR lasers is that in the DFB laser the wavy interface lies between the laser active layer and a waveguide layer. In the DBR laser the wavy interface follows the laser-active layer between the waveguide layer and another layer having a lower refractive index. The prior art DFB and DBR lasers emit extremely narrow-banded light of a single longitudinal mode. Therefore, these lasers are suitable for optically coherent data transmission as well as for optical data transmission at high bit rates over long paths.
The prior art DFB and DBR lasers have the drawback that they can be tuned only over a very narrow range, for example up to .DELTA..lambda.=2 nm and therefore can be used only conditionally for wavelength multiplex operation. Tuning is effected by increasing the free charge carrier density in the laser-active layer or by a change in temperature.